1. Field of the Invention
The present invention relates to a method for anisotropically etching a wafer by means of a plasma, in particular but not exclusively anisotropically plasma etching a deep etch feature into a semiconductor wafer.
2. Description of the Related Art
Etching deep features, or “through wafer” features such as via holes, in semiconductor wafers such as silicon have been achieved using the so-called “Bosch” process. The Bosch process was developed by Laermer and Schlip [1,2,3,4] at Bosch Routington Germany.
Referring to FIG. 1, the process etches deep or “through” structures 14 in a silicon wafer 10 in a pattern defined by a mask 12, such as a photoresist layer, using cyclic etch and deposition steps. The etch step uses a sulphur hexafluoride (SF6) plasma, some times with a small amount oxygen (O2), and then a deposition step using octofluorocyclobutane (C4F8) for the deposition of a fluorocarbon polymer (CF2n). The polymer is deposited on all surfaces of the etched feature. The sidewall deposition of the polymer is important for maintaining the anisotropic character of the etched feature as it inhibits etching of the sidewall thereby reducing under-etching.
During the etch cycle fluorine ions and free radicals (responsible for the lateral etching) in the plasma remove the polymer which has been deposited on the base or bottom wall of the etched feature exposing the silicon to more etching at the base or bottom wall whilst the polymer deposited on the sidewalls inhibits and reduces etching by free radicals. Just before the side wall deposition is removed the etch step is stopped and the deposition step starts again. The etch and deposition step is called a cycle and many cycles are required to etch through the silicon wafer. The etch features are formed through a plasma mask such as a polymer resist or oxide mask.
The known process forms an uneven surface “scalloping” 16 on the side wall of the etch feature 14 due to the cyclic nature of the process as illustrated schematically in FIG. 1 and shown in the micrograph shown in FIG. 2.
FIG. 2 shows an end view of a trench having an oxide mask 22, and shows that even when the scalloping 24 is small the sides 26 are not smooth. The process is sensitive to many parameter changes which may make it difficult to etch features accurately and reproducibly.
In the Bosch process wafer temperature control is very important as it affects the polymer thickness, free radical activity and mask selectivity. Lower temperatures have a benefit of improving the mask life but can promote more polymer growth on the wafer base, such excessive polymer formation causing so-called “grass” formation at the base of the etched feature. Typical grass formation 30 is shown in the micrograph of FIG. 3 where the temperature of the electrostatic chuck holding the wafer has changed from 10° C. to −10° C. Low temperatures can also cause thick resist masks to crack.
Lowering of surface temperature increases the CF2n sticking coefficient which also increases polymer growth and not only increases grass formation, but can cause profile control problems. Other changes in profile control are from changes in free radical loading such as changes in exposed silicon etching area and feature size. For example, small features etch slower than large features as there are fewer radicals available due to the collimation effect of the features on the free radicals.
Ions entering the dark space sheath of the etching plasma are accelerated by the sheath field and impact onto the substrate at an angle substantially normal to the wafer plane increasing the anisotropic etching. Free radicals enter the sheath with a wide spread in distribution. As the etched feature becomes deeper the collimation affects the etch/deposition balance so the process needs to be changed during the wafer etch. Typical profile problems 40 are shown in the micrograph of FIG. 4.
Although it is possible to correct profile problems by parameter ramping control to change the balance of the etch/deposition ratio whilst etching through the wafer, such an approach requires extensive tests to establish profile control and can cause further roughing of the side wall.
The Applicant has recognized that many of the problems with the Bosch type process may be solved if the polymer was only deposited on the side wall and the process was continuous, i.e. none cyclic, which will not form scallops or side wall roughening, and that it is far better to develop continuous stable plasma etch process.
One method attempted by the Applicant was to help reduce scalloping for smoother side walls by using short cycle times less than 3 seconds which produces small scallops. This gives system control problems of power, pressure, and flow as the move to lower residence times results in unrepeatable runs often from matching network and flow response being too slow for these steps.
The Applicant has tried the foregoing approach with a high oxygen addition to a SF6 and C4F8 discharge. Here the O2 produced O+ ions which removed the polymer at the base. However the O2 free radicals were found to be very reactive and dissociate the C4F8 too much and also removed the side wall polymer causing severe undercut. In addition the oxygen oxidizes the silicon surface reducing the etch rate to the fluorine radicals and ions.
Aspects and embodiments of the present invention were devised with the foregoing in mind.